* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download ASTRO-114--Lecture 38-
Survey
Document related concepts
International Ultraviolet Explorer wikipedia , lookup
Corona Borealis wikipedia , lookup
Auriga (constellation) wikipedia , lookup
Aries (constellation) wikipedia , lookup
Canis Minor wikipedia , lookup
Stellar evolution wikipedia , lookup
Cassiopeia (constellation) wikipedia , lookup
Canis Major wikipedia , lookup
Star formation wikipedia , lookup
Star catalogue wikipedia , lookup
Corona Australis wikipedia , lookup
Stellar kinematics wikipedia , lookup
Perseus (constellation) wikipedia , lookup
Cygnus (constellation) wikipedia , lookup
Cosmic distance ladder wikipedia , lookup
Observational astronomy wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Transcript
ASTRO 114 Lecture 38 1 Okay. We’re gonna continue our discussion of stars today. Yesterday we talked about the names of stars. We talked about measuring distances to stars. Today we’re gonna start discussing other properties of stars. In particular, I want to begin discussing the brightnesses of stars. If you look up at the sky at night, it’s pretty obvious right away that not all the stars are the same brightness so we have to have some way of describing their brightness. We actually use two different systems to describe brightness. One system, which is the older one, talks about the way things look. In other words, if you look up at the sky at night, some stars look brighter than o thers. We refer to that as apparent brightness, how they appear. And so historically that was the brightness that was discussed first, the apparent brightness. We have to have some scale in order to describe that brightness. I could describe the brightness of a star as being as bright as a candle seen 10 feet away, if I wanted to, or I could describe the brightness of a star as being as bright as a firefly. You have to have some way of describing it, comparing it with something else. So what astronomers did 2000 years ago was just to label stars with numbers that more or less described their brightnesses. So, for example, the brightest stars in the sky — if you look around the sky, there are about 20 stars that really jump out at you. They are the brightest. I mentioned Rigel and Betelgeuse. They’re both very bright stars. They were referred to as the first magnitude stars, first meaning first place, best, most important. We use that term today when we say first class, the most important things. So first magnitude meant most in ASTRO 114 Lecture 38 2 importance because it was assumed that the brightest stars were the most important ones. And so they were labeled as first magnitude. And, as I said, there are about 20 of them all around the sky. Then astronomers labeled a second group of stars as second magnitude. So you might think of those as second class stars, not quite as important but still quite bright. If you’re trying to think of how bright they would be, the Big Dipper stars are almost all second magnitude. So if you’re looking at the Big Dipper in the sky at night, you’re seeing second magnitude. Then there was the next group, not so obvious, which was labeled the third magnitude. So you might think of that as third class. And those are stars that are making up parts of constellations but they’re not the most important stars. Now, some of them actually are quite important. Polaris, the pole star, is actually a third magnitude star. It’s not bright enough to be first magnitude or second magnitude, and that’s quite an important star. So it’s still visible; it just doesn’t jump out at you in the sky. So those were the first three magnitudes. And then there were fainter stars, stars that you would see that sort of just randomly distributed around the sky. You might not notice them unless you actually focused on a particular area of sky and those were labeled fourth magnitude, fifth magnitude. And then the faintest stars that were visible to the naked eye that you could see on a dark night outside where there were no lights, they were labeled sixth magnitude. So there were six groups of stars. First through sixth, and that pretty much covered everything you could see. It was a simple scale and anybody could go outside ASTRO 114 Lecture 38 3 and look up at the stars. And after a little practice, they could pick a star and tell what its magnitude was. Because you only had six numbers to choose from and usually you could tell whether it was first, ‘cause they were very bright, or whether it was sixth, because you could hardly see them, and then everything in-between kind of fell into place. That system was set up about 200 B.C. by Hipparchus, a Greek astronomer. And because it was simple and easy to use, and it made sense that the most important stars would be labeled one and the next most important would be labeled two, and so on down to six, astronomers began to use it pretty much universally. So for 2000 years almost every star chart that was made had the same labeling on it for magnitudes. And up until the 1700/1800s, it was fine. Because most stars that anybody ever talked about were the stars visible to the naked eye and they were labeled from one to six. So we could always talk about brightnesses of stars by these simple numbers. Then things got a little more complicated. Galileo had to go use the telescope in astronomy and that meant that when he looked through the telescope, he could see fainter stars. He could see stars that were not visible to the eye. Well, what was he gonna call those? Well, he just continued the scale. Six was the faintest visible to the eye and so he just labeled the next group as seven. So there was a seventh magnitude. And then there were even fainter ones and he labeled those eighth. And then when people began to build larger telescopes and they could see even fainter stars than eighth, they labeled them ninth and tenth. So you just keep adding to the numbers. By the time Herschel came along with his big 48-inch mirror telescope, he was looking at fourteenth and fifteenth magnitudes because he kept extending the scale. You ASTRO 114 Lecture 38 4 start off with one, you go down to six as far as the eye is concerned, and then using a telescope you see seven, eight, nine, ten, and so on. The problem is that nobody had really set this scale up accurately. This was just eyeball estimates. When you’re looking through a telescope, how do you know you’re making a good eyeball estimate? I mean, you’re looking through a telescope. It’s not even an eyeball estimate. It’s a telescope estimate. And so Herschel was concerned that he might be calling a star fourteenth magnitude but somebody else might call it twelfth magnitude. Well, who was to agree? How do you decide how bright it really is and what number it should get? And so astronomers realized they had to make this system a little bit more scientific. They had to nail down what they meant by these brightness numbers. But historically that wasn’t how it was invented. It was just invented by looking at stars and giving them numbers. And so astronomers d id some measurements. They actually measured how bright a first magnitude star was. In other words, how many photons are coming in from a first magnitude star compared to how many are coming in for a second magnitude and a third magnitude, and so on. That’s a real brightness measurement if you count photons, for example. And so what they discovered to their surprise was that the magnitude scale really didn’t refer to anything in particular. Going from first magnitude to sixth magnitude turned out to be a difference in the number of photons coming in of about 110. In other words, there were 110 times as many photons coming from a first magnitude star as from a sixth magnitude star. That’s no even number. That’s not easy to figure out how you divide it up. ASTRO 114 Lecture 38 5 It was just the way the eye works. The eye does not work very well as a measuring instrument. Our eyes are used to looking at very bright objects. You go outside in the daytime and you can see with the Sun shining and you can go outside at night a nd see with only starlight out. So your eye adjusts to bright and faint, so it’s not a very good light measuring instrument. And so astronomers were just sort of making it up when they made up that magnitude scale. But they wanted to have a more scientific scale. Now, you could take the old scale and just throw it out and start over again. Just talk about photons or how bright these stars are compared to standard candles or, you know, something else. But astronomers are very traditional. As I mentioned, they don’t like to change names for stars so they just keep ‘em all. They don’t like to change whatever they’ve been doing for 2000 years and astronomers had been using magnitudes for 2000 years. They didn’t want to give it up. Every star chart ever drawn had those magnitudes on ‘em. Why start all over again? So they said, “Okay. Let’s mess around with the system and make it work, even though we’ll have to make it a little more scientific.” So what was done was to say, “Okay. Measurements have been made that shows from first magnitude to sixth magnitude was actually a factor of 110 in brightness. Well, we’ll round that off. We’ll round it to exactly 100. We’ll define first magnitude to sixth magnitude as exactly 100 times in brightness. Just define it that way. It’s close to the old way of doing things. It’s not exactly. We rounded it off a little, but that’s good enough.” And so with that basic definition, that going from first to sixth magnitude, ASTRO 114 Lecture 38 6 astronomers were able to nail down the entire scale. Because what they said was first to sixth is a change of five numbers. First to second is one change, second to third is two, third to fourth is three, fourth to fifth is four, and fifth to sixth is five. So a change of five numbers on the magnitude scale is exactly 100 times in brightness. It’s the way it originally started out, it’s how it was defined. But what I mean by that is — let’s say I go from sixth magnitude up to eleventh magnitude. Now I’m extending the scale beyond where it started. I will say — I will define the difference in brightness from sixth magnitude to eleventh magnitude also to be a factor of 100. So a sixth magnitude star will be exactly 100 times brighter than an eleventh magnitude star. I will just define it tha t way. Just the way I did from first to sixth I’ll do from sixth to eleventh. From eleventh to sixteenth magnitude is another factor of exactly 100. And from sixteenth magnitude to twenty-first, exactly a factor of 100. And so now I’ve defined my numbers better. So when Herschel says he’s looking at a fourteenth magnitude star, he knows that it’s so many times fainter than a ninth magnitude star. It’s five magnitudes fainter so it’s 100 times fainter. So there’s a way to go from the old system and yet extend it to fainter and fainter objects. Once you’ve done that, you can actually go to brighter objects. From first magnitude to something brighter than first. Now what do I mean by “brighter than first”? I thought those were the brightest stars. They are. The stars are first magnitude but what about the planets? What about Jupiter? Jupiter’s brighter than the brightest stars. Venus is brighter than the brightest stars. Sometimes Mars is brighter than the brightest stars. And so you’ve gotta b e able to ASTRO 114 Lecture 38 7 go to brighter objects. The Moon is brighter than the brightest stars. Maybe I’d like to talk about the brightness of the Moon on the same scale. Well, how would I do that? Well, that was a little trickier because astronomers started with the number one. And they went to bigger numbers for fainter objects and that would mean we have to go to smaller numbers for brighter objects. But how do you go to smaller numbers if you started with one? Well, you go to zero and then you go to -1, -2, -3. And so when they extended this scale they did it in both directions. They extended it to tenth, fifteenth, twentieth magnitude fainter stars, fainter objects, but they also extended it to brighter objects like the planets, the Moon, and even the Sun. A nd so when you start looking at the numbers for, let’s say, Jupiter, Jupiter is minus second magnitude. So you’re still using that same scale Hipparchus came up with, but you’ve just extended it. Venus is minus fourth magnitude. Think about that. First magnitude is five magnitudes away from minus fourth, right? Because you’ve got zero in there. That means that the planet Venus up in the sky is 100 times brighter than the brightest stars in the sky. But you wouldn’t know that offhand unless we had a scale to label it. Now we have a scale. Venus is minus fourth magnitude. Now we have some understanding of how bright that is. How bright is the Moon? Well, it varies. Depends on what phase it is. You get to a Full Moon and some of you have seen that up in the sky and you know it’s very bright. It’s about minute twelfth, minus thirteenth magnitude. That’s a very negative number because it’s much brighter than any of the other objects. What about the Sun? Where’s ASTRO 114 Lecture 38 8 that on this scale? Well, the Sun is much, much, much brighter than anything else and so you’ve gotta go a whole bunch of magnitudes to get to it. Remember, every five numbers is a factor of 100 more in brightness. The Sun comes out as -26 on that scale. But think about that. I’m able to describe the brightness of every object in the sky from the Sun down to the faintest stars in the sky, and I never have to use a number larger than two digits. Minus 26 for the Sun. The faintest stars seen through the Hubble telescope, which are the ones we’ve seen the faintest so far, are +29. Two digit numbers. And I can describe the brightness of everything from one extreme to the other. So most numbers are astronomical when I talk about millions and billions of light years or something. But with brightnesses, two numbers at most: -26 up to +29. So it’s a very compact scale. But you have to get used to using it because it is compact. And if I say a star is tenth magnitude, that’s a lot fainter than a fifth magnitude star. It’s not five times fainter, it’s 100 times fainter — and you’ve gotta remember that. And so that’s the scale we use even today. Astronomers still use the same number system that was invented over 2000 years ago when we talk about magnitudes. I showed you this chart of the constellation of Orion. I didn’t particularly describe everything that was on this chart because at the time I hadn’t talked about magnitudes, but there are actual magnitudes listed on here, how bright each of the stars are. For example, I mentioned Rigel. We talked about it having the name Beta Orionis. But right next to that is a zero. Why is that? Because Rigel is actually one of the brightest first magnitude stars in the sky and if you actually measure its brightness carefully, it’s brighter than one. It’s close to zero magnitude. And so it’s labeled as, essentially in round ASTRO 114 Lecture 38 9 numbers, zero magnitude. And Betelgeuse is labeled as 0.5. So we can actually get down to decimals when we talk about these magnitudes now because we have defined the scale accurately. 1.0 is actually defined as the average brightness of the 20 brightest stars so we know exactly what that brightness is. Rigel is brighter than average so it’s actually close to zero. Betelgeuse is brighter than average so it’s 0.5. But there are other magnitudes labeled on here. Eta Orionis is third magnitude. Here’s a fourth magnitude. There’s a fifth magnitude. Up at the top of the chart we have a sixth magnitude. You can hardly see it. Because remember, sixth magnitude is the faintest visible to the unaided eye. Seventh, eighth and ninth. Those arrows seem to be pointing at empty space. Why? Because they’re too faint to be seen. But they’re there. They’re just very faint. Through a telescope you would see those stars. And so on here is an idea of how we label these stars just in magnitudes and these are just rounded numbers. But we can actually give magnitudes to two or three decimals if we wanted to and in some cases astronomers do. If they want to know the brightness of an object very accurately, they could say it’s 3.111 magnitude. That means it’s a little bit fainter than 3 but definitely brighter than 4. And so we can give very accurate numbers now because we’ve defined the system carefully. But we’re still using the same system that’s 2000 years old. Now, there’s one minor problem with that. Those apparent magnitudes, apparent brightnesses, are what we see from here, from the Earth. We are looking at stars and that’s all we see. That’s how they look to us. But when I used that scale and I say that the ASTRO 114 Lecture 38 10 Sun is -26, am I really saying that the Sun is millions of times brighter than any other star? It just looks millions of times brighter. But what if I took Rigel and I took the Sun, if I had that ability, and I put them at the same distance from each other and then I looked at both of ‘em. Which one would be brighter then? Which is really brighter as a star? Well, I don’t know that because all I see is what’s up in the sky. But I can calculate it because I know how far away Rigel is because I’ve measured its parallax, and so I know that it’s hundreds of light years away. So I’m cheating if I say the Sun is brighter than Rigel because the Sun is right next -door. It’s only eight light minutes away. Well, what if I put them both at one light year away. Which one would be brighter? You can bet it would be Rigel. Rigel is actually a much brighter star than the Sun but you don’t know that offhand. But if you put them near each other, then you would be well aware of it. And so astronomers have come up with a second magnitude scale, a different one, one that they refer to as absolute magnitude. Notice the difference in wording. Apparent, how it appears; absolute, just using that word means something a little stronger, how they really are. So if I want to talk about how bright every star is in the sky compared to each other, then I’m comparing apples with apples. I’m being fair. But I’ve gotta compare them all at t he same distance and so astronomers had to pick a distance. How far away are we gonna compare them all so we can give them all a number? Well, it was an arbitrary choice. Astronomers just said, “Okay. We’ll pick some convenient distance. We’ll measure and calculate all of ‘em, find out how bright they really are.” That distance chosen was 10 parsecs. Which if you remember the ASTRO 114 Lecture 38 11 comparison between light years and parsecs is about 32-1/4 light years. It’s not a round number in light years, but it’s a round number in parsecs. Ten parsecs away. So we now compare all stars as if they were 10 parsecs away and see how bright they all are. If I could take the Sun and put it 10 parsecs away, how bright would it look? Boy, would you be disappointed. It would be only a fifth magnitude star. You would hardly notice it in the sky. It’d just be a little faint star up there. If you took Rigel and put it 10 parsecs away, it would be brighter than Venus. Because it’s pretty far away right now and it’s still one of the brightest stars in the sky. And so if you actually put it closer, all the way up to 10 parsecs, it’s gonna be even brighter. And so now we start to get a feel for how bright these stars really are. If you can calculate how bright they all are at 10 parsecs, then we can compare one star with another. We do that by saying, “Okay. The sun is close. How much fainter would it be if I put it that far away? Those other stars are far away. How much brighter would they be if I put ‘em up to 10 parsecs away?” And so we adjust all the numbers so that they’re all from 10 parsecs away. Yes? [Inaudible student response] Offhand, I think it would be about -7 or 8. Brighter than Venus. You could almost read by it. It’d be as bright as a quarter Moon. But it’s quite far away so it’s really not that bright as far as we see. And there are lots of other stars. In fact, Betelgeuse, the other bright star in Orion, is the same brightness. It would be about minus eighth magnitude also. So these are extremely bright stars. They just happen to be 1000 light years away and so they look kind of faint. But if you put the Sun 1000 light years away, it’d be invisible ASTRO 114 Lecture 38 12 because it’s actually fairly faint. But since it’s right next to us, it’s hidden that fact for a long time because it looks pretty bright in the sky. And so in order to really understand the brightnesses, the intrinsic brightness — that’s what that terms means up there — the real brightness of stars, we have to calculate their absolute magnitudes. Another term for the intrinsic brightness that the author of your text uses a lot is luminosity. The luminosity of a star is its real brightness. And we usually talk about that real brightness as absolute magnitude. So if I want to know the real brightness of a star, I will say, “Well, what’s the absolute magnitude of that star?” I might also be interested in how bright it looks in the sky. That would be its apparent magnitude. Keep those straight. Now, here’s a scale — I’ve been talking about these magnitudes — just to give you an idea of how it works. On this scale you see from -25 down to positive numbers. I know that’s backwards from the way you normally think, but bright is at the top of the scale. Negative numbers going to positive numbers. Notice the Sun, a little bit brighter than 25. It’s -26. A 100 watt bulb — think about a 100 watt lightbulb; you’ve seen those before — if you could see it 100 feet away. That’s about minus thirteenth magnitude. If you’re trying to figure out how bright they really are, that might be a good experiment. Go home, get a 100 watt bulb, put it 100 feet away, look at it and say, “Okay. That’s almost the same brightness as the full moon.” You wouldn’t think of the full moon being that faint, would you? A 100 watt bulb 100 feet away. But that’s about it. And then you go to much fainter objects. Notice I mentioned Venus, minus fourth magnitude. The brightest star in the sky is Sirius. It’s up in the winter sky. It’s actually ASTRO 114 Lecture 38 13 minus one magnitude. Saturn, about plus one, naked eye limit six, binocular limit about 10. So if you’ve got a good pair of binoculars, you go outside at night and look around the sky, you can see stars down to tenth magnitude. And if you want to know how many of them there are, there are more than a million of them down to tenth magnitude. So you can see a lot of stars with a pair of binoculars. All the way down here’s Pluto, fifteenth magnitude. Actually, most of the time Pluto is around fourteenth so they’re exaggerating here, making it a little fainter than it actually is but still very, very faint. You’re not gonna see it in binoculars, that’s for sure. You’re not gonna see it in normal small telescopes. You need large telescopes to get to twentieth magnitude. Very large telescopes with long photographs or images, twenty-fifth. And, as I mentioned, the Hubble telescope which is not on here is at twenty-ninth. So that’s the whole scale from brightest to faintest. What else do we want to know about stars other than their brightnesses? We want to know how hot they are so we want to determine temperatures for stars. And there are two major ways of doing that. The easiest way is just to look at its color. We’ve already talked about black body curves, plonk curves, objects that hotter, give off more high energy photons so they tend to look bluer. Objects that are cooler give off more low energy photons. They tend to look redder. So I mentioned when we talked about the Sun that the 6000 Kelvin surface of the Sun looks yellow, but the 4000 Kelvin spots, the sunspots, look more red because they’re cooler. So if you just think red as cool, blue as hot, you can have a rough idea of how hot stars are by looking at their colors. Betelgeuse, the bright star we mentioned a minute ASTRO 114 Lecture 38 14 ago, is about 3000 Kelvin. It’s red. If you look at it in the sky, you can see it has a red tint to it. You look at Rigel, Rigel is almost 20,000 degrees. It looks blue — well, bluish. It doesn’t really look blue. It has a bluish cast. It’s almost white with a bluish tint. But it’s definitely a different color from Betelgeuse. And so a quick way of doing temperature estimates is just by measuring colors. Yes? [Student: Does the temperature have anything to do with the brightness of stars?] It could, and we’ll talk about that a little bit later. Not necessarily, but in many cases yes. If you just think of it in logical terms, the hotter I make a lightbulb, the brighter it would probably be. But what you’ve gotta worry about also is how big i t is. Because a very small, very hot lightbulb may still not give out as much light as a cooler, very large lightbulb. It’s a combination of things. Both the temperature and the size of the object determines how bright it really is. So we measure colors. Now, astronomers have gotten very good at measuring colors for stars so we can actually measure very small differences in color. We measure different parts of the spectrum and we see how the black body spectrum changes with different temperatures, and so we can get very accurate — very, within 100 degrees of different temperatures for stars by measuring their colors. But another way of doing it — and one that is used just as often and sometimes more often — is by looking at the spectrum of stars. And that starts a whole new topic because the spectrum of stars is a major discussion. Here we have a spectrum. This could be the star Vega. In fact, it’s probably about the kind of star — no, not really. It’s ASTRO 114 Lecture 38 15 some other star. It’s not Vega. I’m not sure what it is ‘cause it’s not labeled as such. But anyway, this is a spectrum of a star. You see it has lots of lines in the spectrum caused by various elements. This element is hydrogen. We’ve already discussed those lines in the spectrum. Hydrogen here, hydrogen there and there and there. These are two calcium lines. There are some lines here of various metals, very weak ones. So you could go through this spectrum and you could identify the various elements that produce the lines in this spectrum. We can also measure the brightness across the spectrum and plot a graph of the brightness across the spectrum. There are two ways of looking at a spectrum. You can take a picture of it and just look at the lines or you can measure the brightness across it and then plot that brightness as a graph so that a dark line shows up as a dip in brightness, ‘cause it’s black, and a bright region shows up as high on the graph because there’s a lot of light coming through. So there’s two ways of looking at spectra: either looking at a tracing of the brightness across the spectrum or looking at the spectrum itself. In either case, the first thing you notice are the lines. You see dark lines when you look at a picture of the spectrum, you see spikes in the graph w hen you are just looking at the graph. Astronomers began looking at spectra of stars back in the 1800s. They were curious at first as to what they were made of. They had already looked at the spectrum of the Sun and had found hydrogen and helium — in fact, helium was first discovered on the Sun; it wasn’t even known on the Earth -- and calcium and several other elements, and so they wanted to look at the spectrum of stars to see if they looked the same. And so astronomers began to photograph star spectra. Now, back in the 1800s ASTRO 114 Lecture 38 16 the photographs were not in color so you had a black and white picture with lines on it. And, in fact, most spectra that astronomers take is still black and white. This is the kind we would show to students because you can see the colors. But if I were doing research on this star, I wouldn’t bother using color film. I’d just use black and white. Because what I’m interested in are the lines and they show up just as well in black and white, maybe even better than they do in color. So we look at the lines because we’re interested in what elements the stars are made of. Well, when astronomers began looking at spectra they didn’t know what to expect at first. They just took a bunch of photographs and then they studied ‘em, saw whether they were like the Sun or different. And what they found was that most of the spectra of stars were different from the Sun. They didn’t have exactly the same kind of spectrum. They seemed to have different lines in the spectrum indicating, they thought, that the stars had different elements. For example, there were a lot of stars that only showed hydrogen lines. Almost nothing else, just hydrogen lines. There were other stars that showed hydrogen lines and some helium lines, and that was about it. There were other stars that showed hydrogen and helium and maybe some metals. There were other spectra that showed hydrogen, no helium, and some metals. And so there was a variety of different kinds of spectra and so they at first thought stars were made of different things. And so they decided they needed to label these so that they could keep track of ‘em. Astronomers love to label things. They label magnitudes one through six. How are they gonna label spectra? Well, they could’ve labeled them one through ten or one through twenty, but we ASTRO 114 Lecture 38 17 didn’t. We labeled them by letters of the alphabet: A, B, C, D, E, F, G. It’s as good as anything. And so the first spectrum that we labeled was labeled A, right? If you’re gonna start using letters of the alphabet, you start at the beginning. So you label the simplest spectrum A. Well, what’s the simplest spectrum that we’ve seen? Just hydrogen lines. So stars that had just hydrogen lines were labeled A . Stars that had hydrogen and helium lines were labeled B. Stars that had hydrogen, helium and maybe calcium lines were labeled C. And then D. And then E. Down the alphabet. So things got more and more complicated as they went down the alphabet. A fairly simple way of doing it. You look at a spectrum, you see which lines are in the spectrum, and you say, “Ah, only hydrogen, that’s A. Ah, hydrogen and helium, that’s B. Hydrogen, helium, calcium, that’s C,” and so on. Well, after awhile you’ve got a pile of A spectra and a pile of B and a pile of C, and so you start to see that there are this kinds of stars, there are these kinds of stars, there are these kinds, and so on. Well, astronomers were doing that toward the end of the 19th century and the y decided to do a large survey of spectra. Not just here and there, a few stars, but a whole bunch of ‘em. And the reason that they decided on a large survey was because of an astronomer named Henry Draper. Henry Draper was a rare type of astronomer. He was rich. Most astronomers are not rich. They earn a living like everybody else and they worry about their nickels and dimes, but Henry Draper made his money otherwise. He didn’t make it doing astronomy. He had a wealthy family and he liked astronomy, and so he worked at it but he didn’t actually have to make a living at it because he was wealthy. Well, when he ASTRO 114 Lecture 38 18 died, a lot of his fortune was left to Harvard Observatory because that’s where he’d been affiliated. And he left the money specifically for astronomers to study the spectra of stars and to put together a large catalog of those spectra. So since Harvard Observatory had a lot of money to do the study, they figured, “Okay, we’ll do a large survey. We’ll get the spectrum of every star in the sky down to ninth magnitude.” That’s about 350,000 stars. That’s a major job. So in the 1890s they began to make the Henry Draper Catalog named after Henry Draper who left all the money. And the person who was put in charge of taking care of that catalog was a woman named Annie Cannon. Now, back in the 1890s, women were not allowed to be real astronomers. They couldn’t get a doctorate in astronomy. They couldn’t be hired by a university in most cases to work in astronomy, but they could be hired as a ssistants. They were subservient in that way. But there were still many women who were good at astronomy and Harvard realized this. Even though they couldn’t hire them as astronomers, they hired ‘em as astronomers’ assistants. But since they were very good, they really weren’t assistants. They were astronomers without the title. And Annie Cannon was given the job of taking care of the Henry Draper Catalog. She was in charge of it. So she had a fairly large budget to take care of this catalog. She personally examined almost every spectrum that ever was looked at in the catalog. Remember I said there were 350,000 stars. She looked at almost all of ‘em. And so she was the one that would look at the spectrum and say, “Ah, that’s A” or “That’s B” or “That’s C” or “That’s K” or “That’s M.” She was the one that put the letters on those spectra. ASTRO 114 Lecture 38 19 And when she began doing this full-time, she realized there was a problem. There was something wrong with this whole classification. It was messed up. Starti ng with the beginning of the alphabet and going down and making them more complicated was not the right way to go. She realized that they didn’t fit together right. You would think if you go A, B, C, D, E, F, G that you have a nice change in the spectrum as you go from one letter to the other. Not that there’s a random change. And she realized that she could rearrange the spectra so that there was a nice, uniform change from one kind of spectrum to the next, to the next, to the next, to the next, which seemed to make some physical sense. There was something about these spectra that when they were arranged properly, one just sort of became the next, became the next, so there was a nice uniform change. But she realized she had to rearrange the letters ‘cause she had to move ‘em around. And so she rearranged the letters and just about drove everybody crazy. Because they started out in alphabetical order and suddenly she wanted to completely rearrange them like this. Now, it’s a little bit hard to see the details on this drawing, but I think you can see the general spectra. You start out up here with almost no lines and then if you arrange them in this order, you notice that, for example, the hydrogen line gets stronger, stronger, stronger, and then fainter, fainter, fainter. So it kind of flows from one kind of spectrum to the other. You have the same thing here, going from faint to brighter to fainter. You have this line that goes from faint to bright to fainter. And so it looks like they should b e arranged in that order. Well, when she arranged them in that order, notice what happened to the ASTRO 114 Lecture 38 20 alphabet: O, B, A, F, G, K, M. That’s not in alphabetical order. And so she proposed to change the whole system and put ‘em in a different order. O, B, A, F, G, K, M. Way out of alphabetical order. But what order is it in? It’s out of alphabetical order but what order is it in? It turns out it’s in order of temperature. The hottest stars have very few lines. The atmosphere is so hot the elements can’t absorb any photons. And so very few lines. The only lines you can see are helium lines and a little weak hydrogen. When you get to A — remember A was the first letter — mostly hydrogen. Notice that the hydrogen lines are very strong. That’s what we originally thought. Hydrogen lines were letter A. But all these other ones are in some other order because that’s the way they fit together down to M which is the coolest stars. Now, notice there are a lot of letters missing here. Where is C? Where is D and E and H? Turns out they didn’t mean anything. The more she looked at ‘em, the more she realized they were just plain mistakes and that she could put them in the other groups by putting sub-numbers. So, for example, we have here B2 and B5. They’re both labeled B but they’re subsets of B. But they’re all B. And so you get rid of C because you’ve got different kinds of B. You can get rid of D by having a couple of different kinds of F: F0, F5. So astronomers got rid of a lot of the letters, combined ‘em with the letters that were left, put the letters out of order and that’s just to confuse students. They did that on purpose just to confuse students. And you have to memorize it: O, B, A, F, G, K, M. Learn those letters in that order: O, B, A, F, G, K, M. However you want to learn it. Henry Norris Russell at Princeton invented a little ditty: Oh Be A Fine Girl, Kiss Me. ASTRO 114 Lecture 38 21 He’s a dirty old man. But anyway it works. If you say those words — Oh Be A Fine Girl, Kiss Me — it’s in the right order. Or you can see Oh Be A Fine Guy, Kiss Me. So it’s not sexist. Either way.